The present invention is directed to a device and method for sensing and monitoring the concentration level of a material carried in a body of fluid. More particularly, the invention relates to a device and method which senses and monitors the toner concentration within a liquid solution used in connection with an electrographic printing machine.
Electrographic reproduction includes forming an electrostatic latent image upon a recording medium and subsequently making the latent image visible. The recording medium is usually provided in web form, and has a dielectric and a conductive surface. The medium may be a coated paper, a polyester based transparent film, or other suitable material on which an electrostatic latent image is formed by means of a plurality of writing electrodes physically positioned on one side to electrically address the dielectric surface as the medium travels through a recording station. Opposite the dielectric surface of the recording medium there is a conductive surface which is selectively grounded. When the potential difference between the conductive surface and the recording elements is raised above a threshold level, on the order of several hundred volts, an electrostatic charge is deposited on the dielectric surface of the recording medium as the medium passes by the recording station.
Subsequently, the latent image is made visible at a development station during a development step by applying liquid or dry toner to the recording medium. The recording medium is contacted by a thin film of developer material out of which the toner particles are electrostatically attracted to the regions of electrostatic charge on the medium. These toner particles often are suspended in a liquid solution at a preferred concentration. As more images are developed, the particles suspended in the liquid become depleted causing the concentration of the particles in the liquid to be reduced. Therefore, as will become apparent, it is important to monitor the depletion of these particles as the concentration of the liquid changes, and to compensate for such depletions as they occur.
Electrographic machines, such as electrostatic plotters are available as monochrome machines, including a single recording station and a single development station dispensing a single color toner, usually black. Also, electrostatic color plotters are available which produce full color plots by the sequential overlaying of a series of separate color images (yellow, cyan, magenta and black) to produce a full spectrum of colors.
There are three basic approaches to color separation imaging. In the first, a series of images are formed sequentially each by means of a dedicated recording head and development station. In the second, a single recording head forms each color separation image on the recording medium which is then advanced past one of several development stations. Then, the recording medium is returned to the recording head for receiving the next color separation image and is advanced to the next development station. This process of advancing and returning the recording medium through the apparatus minimizes the number of recording heads and obviates the need for their critical alignment with respect to one another.
The third approach to color separation imaging uses a single recording head to form each color separation image on the recording medium, as in the second approach described above, however only one toner fountain is used for development whereby all toners pass through the same fountain which is purged between colors.
Each of the foregoing systems need to compensate for changes in toner concentration. In liquid toner electrostatic plotters, toner concentration is commonly measured optically. The liquid toner is pumped between two closely spaced, parallel, clear windows, forming a thin layer through which light is passed by an emitter, such as an LED. Toner concentration is determined to be proportional to the amount of light registered at an optical sensor. For example, the higher the number of particles in a fluid the more the light passing to the sensor is blocked. A full description of such a system is detailed in U.S. Pat. No. 4,222,497 to Lloyd et al. which is assigned to a common assignee and hereby incorporated by reference. Various other systems using this approach are described in U.S. Pat. Nos.: 5,319,421; 4,981,362; 4,660,152; 4,166,702; 4,119,989; 3,807,872; 3,712,203; 3,698,356; 3,677,222; and 3,354,802. Typically, color electrographic systems have four such windows, one for each color (e.g. black, cyan, magenta, yellow). U.S. Pat. No. 5,319,421 to West describes a concentrate sensor which has two different sized windowing areas used to compensate for the difference in the optical properties of different color toner solutions. West also discloses self-calibration method for measurement of toner concentrate.
The accuracy of concentration measurement is highly sensitive to the thickness of the toner layer, i.e. the "window thickness", as well as variations in the optical properties of the toner being measured. This requires very tight tolerances on the window (e.g. 20.+-.1 mil) which, realistically, can only be met by sorting parts. A common technique is to calibrate a machine during its manufacture to compensate for the initial properties of the elements being used. This type of calibration process does not, however, take into account element properties that change or degrade over time. In addition, any improvements in toner formulation which affect optical properties cannot be easily implemented.
As technology moves forward and new toners are developed, existing concentration sensors are becoming ineffective in the sensing of color extremes, i.e. black and yellow. For example, a situation is developing where, due to the tight tolerances that are required, and the differences in the black toner solutions, it is difficult to construct a window thin enough to accurately measure these solutions.
To compensate for the deficiencies in concentrate sensors, manufacturers are turning to higher-capability emitters and detectors. These devices are of course more costly which results in a more expensive machine. Another manner of compensating for the deficiencies described above, is to include a circuit which amplifies the detected signal. However, a drawback with such a solution is the amplification of noise such as ambient light and reflection noise. These undesirable amplifications are significant enough to effect readings.
Another drawback with existing concentrate sensors is the need for multiple sensor windows for each of the colors (e.g. black, cyan, magenta, yellow) or the requirement of additional emitters and detectors and a complex multi-level window area such as necessary in the patent to West.
Still a further drawback has to do with calibration of the concentrate sensor. Particularly, while it was noted that calibrations could be performed during machine manufacture, this does not take into account the calibrations necessary or desirable when new toners are added to the machine or the effects of time on components. In these existing machines, it is often necessary for a technician to manually "tweak" the amount of light going through the emitter. While West does describe a self-calibration technique, the implementation of it, is complex requiring use of CPU time and the generation of a variety of look-up tables.
Another drawback with existing concentrate sensors is their inability to be easily implemented within machines having different characteristics, such as between a single toner machine to one with four color toners. This is a matter of significant concern since machines having five and six toners are now under development. Under the present technology such machines would therefore require additional sensor windows or windows of greater size to be implemented, thereby raising the cost and complexity of these machines.
It has been determined to be advantageous to have a variable concentrate sensor with a variable sensor window and emitter and detector which can measure the density of a range of colors without the requirement of increasing the window size and/or requiring additional sensor windows and/or associated emitters and detectors.
It has also been considered valuable to provide a variable concentrate sensor which can measure toners, such as different types of black toner that can have widely varying characteristics. It is also considered valuable to have a variable concentrate sensor which eliminates the complexity of recalibration when new toner is added to a machine. A concentrate sensor of such construction is also considered valuable for calibration as it eliminates the need of a technician and/or the acquisition of information and operations needed to generate look-up tables.
Still another desirable aspect of a variable concentrate sensor is one which allows a larger mechanical variance in the construction of the sensor windows, thereby decreasing the number of non-usable manufactured parts.